Yes we are our brothers keeper.
And no that does not mean we are supposed to have open borders.
God gives extensive law about borders, trespassing, and landmarks in His word. It is never implied that these are to be disrespected. Rather He forbids them being disrespected.
"Keeper" comes from "keep" - a sort of corral or pen. Think "zookeeper" or the "keep" of a castle.
Cain was mockingly asking God the rhetorical question: "Should I be holding my brother in a 'keep' and looking after him the way a person might 'look after' a colony of rabbits?!"
No, our brothers are not animals that we should pen up and watch over like animals.
Regards,
If I were your "keeper" (or your "care-taker" or "legal guardian" [because you had, e.g., been declared by a Court of Law to be mentally incompetent]), wouldn't I have a right - nay, an obligation - to see to it that you got enough bed rest, didn't eat too much, didn't indulge in any excessive drinking, etc. - sort of like how I treat my 9-year-old son?
Wouldn't I, as your "keeper," be entitled to curtail your seeing your friends, if I thought they were a bad influence upon you? Etc.
And how could we all be each other's keeper - i.e., how could it possibly be a reciprocal relationship?
It is a logical impossibility.
Even substituting the word "help-mate" in place of "keeper" is problematic - God ordained only that "man and wife" would (have to) be that to each other.
Regards,